A highly saturated nitrile group-containing copolymer rubber, as represented by hydrogenated acrylonitrile-butadiene copolymer rubber, draws attention. The highly saturated nitrile group-containing copolymer rubber is superior in heat resistance, oil resistance, ozone resistance, etc., compared to common nitrile group-containing copolymer rubber having a lot of carbon-carbon unsaturated bonds in a main chain structure such as acrylonitrile-butadiene copolymer rubber. However, depending on an amount of nitrile group and content of unsaturated bonds in carbon-carbon bonds, it is liable to be inferior in cold resistance compared to a nitrile group-containing copolymer rubber having a lot of carbon-carbon unsaturated bonds. Generally, cold resistance of the nitrile group-containing copolymer rubber can be enhanced by reducing an amount of nitrile group, but when increasing a degree of saturation in a nitrile group-containing copolymer rubber, cold resistance is not always improved by reducing an amount of nitrile group.
It is continuously required to further advance overall performance such as mechanical strength, heat aging resistance, oil resistance, cold resistance and ozone resistance in many fields including car parts. Therefore, it is required to improve cold resistance of highly-saturated nitrile group-containing copolymer rubber.
To solve this problem, Patent Document 1 proposes a highly-saturated nitrile group-containing copolymer rubber, wherein each of α,β-ethylenically unsaturated nitrile unit (a), α,β-ethylenically unsaturated carboxylic acid ester unit (b), conjugated diene unit (c) and saturated conjugated diene unit (d) is included in a specific ratio, and a total ratio of the (c) and (d) and the ratio of the (d) based on the total ratio are set within a specific range, and the difference between the extrapolated glass transition initiating temperature (Tig) and extrapolated glass transition-ending temperature (Teg) is 10° C. or lower. However, while vulcanizate obtained by vulcanizing this rubber can be improved in cold resistance, oil resistance, dynamic characteristics, etc., heat resistance and ozone resistance may be insufficient.    [Patent Article 1] The Japanese Unexamined Patent Publication (A) No. 2001-114940